Thousands of demonstrators have taken to the streets of Moscow to protest against alleged fraud in Sunday's parliamentary elections.

The protest in central Moscow was one of the largest opposition demonstrations in years, with an estimated crowd of between 3,000 and 5,000 people.

Police said they detained 300 people after confronting the crowd who were chanting "Revolution!" and "Russia without Putin".

Police scuffled with some protesters and formed a line to hem them in and prevent them marching towards the Kremlin. Some managed to break away and head towards the seat of power, but at least 30 were seized before they got there.

The protests came as US secretary of state Hillary Clinton raised "serious concerns" about the conduct of the parliamentary elections.

"We do have serious concerns about the conduct of the elections. We think that the preliminary report just issued by the OSCE international mission raised a number of questions about the conduct of the elections," she said.

Opposition parties claim the election was marred by fraud, and observers said there had been "serious indications of ballot box stuffing" in a harsh verdict on the election that suggested United Russia could have suffered an even bigger decline in support if the voting had been completely fair.

They also said the election preparations "were marked by a convergence of the state and the governing party, limited political competition and a lack of fairness."

"The country has never seen such a dirty election," said Communist Party leader Gennady Zyuganov, who dismissed the official results as "theft on an especially grand scale".

Severe setback

Despite that, the ruling United Russia party, backed by prime minister Vladimir Putin suffered a severe setback at the polls, failing to win a majority of the popular vote.

The Central Election Commission said the prime minister's United Russia party was set to have 238 deputies in the 450-seat State Duma after Sunday's vote, compared with 315 seats in the current lower house.

The result was Mr Putin's worst election setback since he came to power 12 years ago and signalled growing weariness with his domination of Russian politics as he prepares to reclaim the presidency in an election next March.

The prime minister says he brought stability to Russia after the chaos in the years that followed the collapse of the Soviet Union in 1991 and Russians benefited from an economic boom fuelled by high oil prices during his presidency from 2000 to 2008.

But many Russians now complain of widespread corruption and the growing gap between the rich and poor, and an increasing number say they are disillusioned with Putin and his party.

We think that the preliminary report just issued by the OSCE international mission raised a number of questions about the conduct of the elections.

US secretary of state Hillary Clinton

Mr Putin, 59, defended the party's performance at a government meeting, saying a simple majority of 226 was enough to pass most legislation, and suggested this was sufficient to maintain stability.

"United Russia has been a significant part of the foundation of our political stability in recent years, so its successful performance in the election was important not just for the government but, in my view, for the whole country," he said.

But Mr Medvedev, who led the party into the election at Putin's behest, said voters had sent "a signal to the authorities" and hinted that officials in regions where the party did badly could face dismissal if they do not shape up.

"United Russia did not do too well in a series of regions, but not because people refuse to trust the party itself ... but simply because local functionaries irritate them," he said.

"They look and they say ... 'if that's United Russia, there's no way I'm going to vote for him'."

Although Mr Putin is still likely to win a presidential election next March, the result could dent the authority of the man who has ruled with a mixture of hardline security policies, political acumen and showmanship.

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